Plant-available potassium assessment with a modified sodium tetraphenylboron method

The conventional 1 M NH 4 OAc-exchangeable potassium (K + ) soil test is inadequate in soils where nonexchangeable K + contributes significantly to crop nutrition. Studies were conducted (i) to compare the abilities of the I M NH 4 OAc method with a modified NaBPh 4 method to estimate critical soil K + levels, (ii) to estimate the contribution of nonexchangeable K + to plant-available K + , (iii) to compare the abilities of the 1 M NH 4 OAc method and the modified NaBPh 4 method to estimate plant dry matter yield and plant-available K + , and (iv) to compare the abilities of both methods to measure soil K + balance. Winter wheat (Triticum aestivum L. 'Abe') was grown in eleven Midwestern soils in a greenhouse using consecutive 28-d defoliation and regrowth cycles. Soils also were incubated for 6 mo with five K + rates (0-809 mg K + kg -1 ). Ammonium acetate- and NaBPh 4 -extractable K + (5-min extraction period) were determined in soil samples taken after every three defoliation cycles and after incubation. Critical soil K + levels could not be determined by either method alone but could be predicted by including cation-exchange capacity (CEC ) and illitic K + content in regression models. Nonexchangeable K + represented a significant portion of plant-available K + . Plant-available K + and dry matter (DM) yield were well related to NH 4 OAc-extract-able K + only in soils with low nonexchangeable K + contribution (r 2 -0.889 and 0.915, respectively), but they were well related to NaBPh 4 -extractable K + in all soils (r 2 = 0.984 and 0.874, respectively). Slopes for NH 4 OAc-extractable K + vs. soil K + balance varied widely among soils (0.16-0.68) depending on NH 4 OAc-extractable K + , illitic K + ,and clay content, but for NaBPh 4 -extractable K + slopes were near unity. These studies suggest that the modified NaBPh 4 method may be a superior K + soil test compared to the NH 4 OAc method. Illite content and CEC data may help in developing better soil K + management guidelines.